This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Chemotaxis, the ability of the cell to sense and move in the direction of higher concentration of chemicals, is an integral part of immune response. Additionally it plays a key role in wound healing, angiogenesis, and embryogenesis. Dictyostelium discoideum, a model system for eukaryotic cells, is a social amoeba and has been studied extensively over the past twenty years. In our experiments, we probe and quantitatively measure the initial chemotactic response of single Dictyostelium cells by quantifying the localization dynamics of this key component of signaling transduction network in response to repeated spatio-temporal pulses of chemoattractant. We find that the response of a single cell is very reproducible from pulse-to-pulse. In contrast, we observe a large variability in the chemotactic response from cell-to-cell even when different cells in population are exposed to the same pulse. Although on average a population of cells finds the correct direction of the pulse, a significant variability is observed in the direction and the magnitude of the response. Origins of the noise and cell individuality by quantitatively are explored by measuring the external concentration of the cAMP molecules. We observe that the reliability in the directional sensing mechanism is not limited by the low number of external cAMP molecules and the noise does not decrease when the external cAMP molecules increases by 2 orders of magnitude. Additional studies aimed at better understanding the chemotaxis mechanism will utilize microfluidic devices produced via soft lithography techniques to generate a variety of spatio-temporal chemical gradients.